Electrostatic screen printing and cleaning

ABSTRACT

An electrostatic screen printing device having means for depositing toner on a substrate and having means for neutralizing the electric field associated with the toner-image. Additional means are provided for cleaning toner from the screen and/or printing device subsequent to a printing operation.

United States Patent 1191 Rarey etaL' I v ELECTROSTATIC SCREEN PRINTING AND CLEANING Inventors: Kenneth W. Rarey, South Holland;

John B. Kennedy, Jr.', Chicago, both of I11.

Assignee: Continental Can Company, Inc.,

New York, NY.

Filed: Feb. 17, 1969 Appl. No.: 802,747

Related US. Application Data Continuation of Ser. No. 599,822, Dec. 7 1966, abandoned, which is a division of Ser. No. 396,060, Sept. 14, 1964, Pat. No. 3,306,193.

U.S. c1....;.....101/114, 10l/DlG. 13, 78/66 A, 346/74ES, 117/175, 555/3 DD, SS/DIG. 1,

Int. Cl B4lf 15/00, BOSb 5/02 Field of Search 101/1310. 13, 114, 129; 65/15 1111 3,824,924 I 451 'Ju1 23, 1974 [56] References Cited UNITED STATES PATENTS 2,576,047 11/1951 Schaffert 101/D1G. 13 2,618,551 11/1952 Walkup 13 2,618,552 11/1952 Wise 13 2,638,416 5/1953 Walkup 13 2,940,864 6/1960 Watson 13 2,965,573 12/1960 Gundlach 101/D1G. 13

Primary Examiner-Robert E. Pulfrey Assistant ExaminerE. H. Eickholt Attorney, Agent, or Firm-'-Diller, Brown, Ramik &

l-lolt [57] ABSTRACT An electrostatic screen printing device having means for depositing toner on a substrate and having means for neutralizing the electric field associated with the toner-image. Additional means are provided for cleaning toner from the screen and/or printing device subsequent to a printing operation.

16 Claims, 8 Drawing Figures PATENTEfi 3.824.924

INVENTORS KENNETH W. RAREY 8 JOHN B. KENNEDY, Jr.

RNEYS afpi gmented or dyed substance which generally is a good electrical insulator, has a low melting tempera- CLEANING CROSSREFERENCES TO RELATED APPLICATIONS was a division of application Ser. No. 396,060, filed Sept. l4, 1964, now US. Pat. No. 3,306,193.

I BACKGROUND OF THE INVENTION f In a previous known method of .electrostatic screen printing, a stencil-supporting, electrically conductive 1 ELECTROSTATIC SCREEN PRINTING AND ,2 A v ing the functions permits control of each independently of the other. b A

The manner in which toner is introduced through the stencil apertures is of fundamental importance. Brushing toner through has several unique characteristics: l) the toner comes through the screen at low velocity which limits the object-screen separation; (2) at any instant, the toner comes through the screen over a limited area, thus limiting printing speed; (3) the uniformity of the toner distribution, in passing through the stencil, is determined by the distribution of toner in the brush and by the constancy of the brushing force and motion. By contrast, the present invention: (I) sends toner through the stencil at high velocity; (2) prints screen is locatednear and paranel to coextensive over an extended area (the entire pattern isprinted sisheet electrode. The screen and electrode may be either planar or curved into some simple shape, such as I a portion of a'cyclinder of a cone. An object to be printed is located between the screen and sheet electrode. These must be sufficiently separated and appro- Though desirable, such a field is not absolutely necespriately shaped to accommodate inclusion of the object therebetween. A high voltage, direct current power supply is connectedbetween the screen and sheet elec-' trodeto establish and maintain an intense electric field therebetween. The screen and sheetelectrode essen- I The ink used in this previouslyknown printing process is afmely-divided powder. This powder is made of ture, and is known as toner. The tonergis applied to the side of thescreenv opposite'to, that facing the objectto be printed. This applicationof toner to the screen may be donewith a roller similar to the type used for painting. 'As the toner is pushed through unblocked apertures in the screen, thetoner acquires a charge from the screen. The toner then accelerated bythe electric I field between the screen and sheet electrode, and is moved toward the sheet electrode until intercepted by the object being printed. After a sufficient amount of toner has been deposited, the flow is interrupted. .The

object may be then removed from its location between the screen and sheet electrode and may be exposedto heat or an appropriate solvent so asto produce permanent image adhesion't'o the object.

' The invention to be disclosed hereinafter exhibits some similarityto the prior art just described. They are both stencilling processes, the stencils are preferably electrically conductive and are generally supported on conductive screens, both' processes can use the same toner particles, which toner particles are electrically charged for both processes, intense electric fields are used to transport toner through space to result in printing, they are both suited to the same printing tasks, and the toner may be permanently affixed to the object or substrate being printed by identical methods.

Various differences also exist between the prior art and the presentinvention. The previously described method uses the conductive screen which supports the stencil for two functions: (1) charging the toner and (2) establishing and maintaining the electric field 'whichcarries the toner to the object to be printed. The

invention to be hereinafter described, provides toner chargingprior to passage of toner through the screen and without contact of toner with the screen; separatmultaneously); and (3) exhibits very good uniformity of toner distribution over extended areas.

In practicing the prior art, it is absolutely necessary that an electric field exists at the location in space where the toner meets the surface to be printed.

sary in this new approach.

Finally, when printing upon insulating'materials, the accumulation of toner as the image is formed produces an electric field which opposes further deposition. It is desirable that charge be added to the object to be printed to minimize this field. The present invention considers and makes allowance for this phenomenon; the prior art methods exhibit a deficiency in this regard.

SUMMARY OF THE INVENTION In general, the invention to be hereinafter disclosed is characterized by: (1) speed; (2) uniformity over ex- BRIEF DESCRIPTION OF THE DRAWING FIG. 1a is a diagrammatical illustration of a developer particle, of the type used in electrostatic printing, of small toner particles adhering.

and shows a plurality to a larger carrier particle.

,FIG. lb is a diagrammatical illustration of fundamental apparatus for performing an electrostatic screen printing operation, and showsa developer particle disposed upon a base electrode in spaced'relation to a conductive stencil screen which is disposed in parallel relation to the base electrode and is connected in an electrical circuit therewith such that an electric field may be produced therebetween.

FIG. 1c is a diagrammatical illustration of the printing apparatus upon closing of the switch to establish an electric field, and illustrates a developer particle being expelled from the base electrode and attracted toward the stencil screen.

FIG. id is a diagrammatical view of the printing apparatus and illustrates the carrier particle as being stopped by the stencil screen while smaller toner particles are carried by their momenta through apertures in the stencil screen.

FIG. 1e discloses a substrate being disposed in the path of the toner particles, and also illustrates the carrier particle as being expelled from the stencil screen and attracted toward the base electrode.

FIG. 1 f is a diagrammatical illustration of the printing apparatus and shows the toner particles, which have passed through the stencil screen, as adhering to the substrate, and also shows the carrier particle on a return flight from the base electrode toward the stencil screen.

FIG. 2 is a diagrammatical view of a printing machine embodied in the present invention.

FIG. 3 is a diagrammatical view of a printing machine embodied in the present invention and illustrates apparatus for and one manner of cleaning the apparatus subsequent to a printing operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing in detail, FIG. la discloses 'a developer particle, generally indicated by the numeral l0, and includes a large carrier particle 11 and a plurality of relatively small toner particles 12 which adhere to the carrier particle 11 by triboelectric forces. The carrier particle 11 may be formed of electrically conductive material, such as charcoal, or metal shot of steel, aluminum, copper, etc.; however, in addition to being electrically conductive, it is preferred that the carrier particle 11 be formed of a ferromagnetic material such as iron filings; The toner particles 12 are preferably formed of a non-conductive material located in a triboelectric series such that the toner particles have a negative charge relative to the carrier particles 11,

v cleswhich are formed of n-butyl methacrylate (41%) and polystyrene (59%). Such toner particles are suppliedby Xerox Corporation for use in electrostatic office copiers and have an average particle diameter of 17 microns.

FIG. lb illustrates a single developer particle as I being disposed upon a base electrode 15 which is formed of conductive material. Located above the base electrode 15 is a stencil screen 16 which is comprised of a wire mesh 17 having a conventional type of coating 18 to define apertures 19 arranged in a predetermined pattern. One commercially available screen which is useful in the present invention is a 200 X 200 mesh stainless steel screen woven at 1.6 mil wire. Stencils conventionally used in silk-screen printing, such as carbon tissue, have also been used.

The base electrode 15 and the stencil screen 16 are connected by suitable conductors to opposite sides of a suitable high voltage source 22 and a suitable switch 23 is provided in one of the conductors. It is preferred, as shown, that the stencil screen 16 be connected to the positive side of the high voltage source 22, while the base electrode 15 is connected to the negative side of the high voltage source 22. The polarities may be reversed from those shown; however, the polarities shown are preferred for reasons which will become apparent from subsequent description. If the toner is positively charged with respect to the carrier, the stencil screen 16 is preferably connected to the negative side of the high voltage source 22.

As is shown in FIG. 1c, closing of the switch 23 establishes an intense electric field between the base electrode 15 and the stencil screen 16, and the developer particle 10 has acquired a net charge from the base electrode 15 and is expelled from the base electrode 15 and is forced by the electric field toward the stencil screen 16. The net charging of the developer particle 10 occurs by direct electrical contact between the conductive carrier particle 11 and the base electrode 15. The conductive carrier particle 11 acquires the same charge as the conductive base electrode 15 and is, therefore, repelled therefrom. FIG. 1d illustrates a sequence of events which occurs at the time that the developer particle 10 is in contact with the conductive stencil screen 16. During the upward movement of the developer particle, as is shown in FIG. 10, the developer particle 10 acquires a velocity such that upon collision of the carrier particle 11 with the stencil screen 16, toner particles 12 which are in alignment with an aperture 19 continue through the stencil screen 16 and rise thereabove. The carrier particle 11 is too large to pass through the small apertures in the stencil screen 16 and, because of contact with the conductive stencil screen 16, the carrier particle 11 acquires a charge at the screen which is opposite to the previous charge upon the carrier particle 11 and, therefore, is repelled by the stencil screen 16 and attracted toward the base electrode 15. An electric field may also be established between the stencil screen 16 and the substrate 25 by providing a back electrode or control grid, such as the grid 41 which is shown in FIG. 2, adjacent to the substrate 25 on the side thereof which is remote from the stencil screen 16. Such a back electrode or control trid is held at the same polarity but at a higher potential than the stencil screen 16. As a result, such an electric field aids in propelling the toner particles toward the substrate 25.

FIGS. 1e and If each illustrates a substrate 25 as being disposed above the stencil screen 16. After the impact of the carrier particle 11 with the stencil screen 16 has released some of the toner particles 12 from the carrier particle 11, momenta of the toner particles carry them into contact with the substrate 25 and the toner particles 12 adhere thereto. During the printing operation, the developer particles 10 continue to oscillate back and forth between the base electrode 15 and the stencil screen 16 until sufficient toner has been acquired upon the substrate 25 to produce a print of the desired optical density. The substrate 25 and the pattern thereon, which is formed by the toner particles 12 being deposited in a pattern which is determined by the apertures 19, are exposed to heat or vapors of a suitable solvent such that the toner particles are fused to the substrate 25.

Illustrated in FIG. 2 is a basic printing machine, generally indicated by the numeral 28, which printing machine embodies the present invention. The printing machine 28 includes a housing 29 which is formed of a non-magnetic material, such as plastic, wood, or any other suitable material. An endless conductive belt 30 is disposed adjacent to the top of the housing 29, and performs the functions as described above with refergenerally indicated bythe numeral 32.

I ence to the base electrode 15. The housing 29 also includes a portion thereof defining an inclined plane 31 which provides a path for cascading developer particles and/or carrier particles 11 downwardly toward a portion of the housing 29 which defines a depository,

ter which drum was covered by wrapping the entire curved surface with butting pieces of a flexible magnetic tape one inch wide and l-/16th inch thick, No. 60,208, purchased from the American Science Center of Chicago, Illinois. This was then wrapped-with mylar film, having athickness of about 40 mils, so as to reduce the field intensity.

The conductive belt electrode 30 is suitably mounted upon a pair of rollers Y36 and 37, either one or both of which are driven by suitable drive means (not shown). The roller 36 is a'magnetic rollerand is disposedadja- 'cent to themagnetic drum 33 and separated therefrom by a portion of the conductive belt electrode 30. The

magnetic roller 36 is designed so as to have a more intense magnetic field associatedtherewith than the magnetic fieldwhichis associated with the magnetic drum 33. Consequently," developer particles 10 or carrier particles 11, which are picked up at the depository 32 by themagnetic drum 33, are transported by the magneticdrum to the conductive belt electrode 30 and, be causeof the more intense magnetic field of the magnetic roller 36, the developer particles -10 or carrier particles 11 are transferred to the surface of the conductive belt electrode 30. As the electrode 30 moves, it carriestheparticles away from' the roller 36 and to vention included a magnetic drum 33 which was com- I prised of an aluminum drum about 12 inches in diamea-doctorblade 38 where the distribution and flow may :be controlled. An overflow-tube (not shown) returns excess particles from the doctoring zone back to the depository 32. Anelectrical connection is generally, though not necessarily, madeto the conductive belt eletrode 30; by going through the doctor-blade 38 and developer particles r After doctoring,-.the developer particles 10 aretransported into the printing zone where the oscillations of r the particles between the electrode 30 and'the stencil screen 16 occur, aswere previously described in FIGS. la-lf. Toner particles 12 which pass through thestencil apertures 19 are electrically transported to the substrate 25 being printed. The electric field which does this is located between the screen 16 and the substrate 25. At the beginning of the printing operation, it is prefassociated with a charged vtoner image which is caused by accumulation of toner pa'rticles 12 upon the substrate 25. Thus, accumulation of toner particles 12 on the substrate 25 has little influence on subsequently arriving toner particles. In the case of a highly conducting substrate, an electrical connection made directly to it can accomplish both functions. For a reasonably conductive object, such as paper, a co-extensive electrode behind it can accomplish both. For these cases, no corona discharges and no control grids are required. But for insulating materials, such as cellulose acetate, the use of the corona discharge is preferred.

' The arrangement described is preferred as a standard approach. For insulating materials, ions deposited on one side and toner particles, carrying charge of the opposite polarity, deposited on the other side of the substrate, tends to form a double charge layer with only a small net external'electric field. For conducting materials, conduction neutralizes the ions but also produces an excess or deficiencyv of electrons at the surface on which the toner particles are deposited, so that a similar result occurs.

After contributing to the printing operatiomthe carrier particles 11 cascade down the inclined plane 31 which returns them to the depository 32. On the way down the inclined plane 31, toner particles 12 are added to replace those depleted by the printing operation. Mounted under the inclined plane is a toner reservoir 45 which is provided with a pivotally mounted cover 46. Inside the reservoir 45 is a cylindrical brush 47, such as a paint roller, which brush is rotatably driven by suitable means (not shown). The brush 47 feeds toner particles from the reservoir 45 through apertures 48 in the inclined plane 31. A preferred con-' As the brush 47 is rotated, toner particles are forced through :the screen apertures 48. As the carrier particles 11 roll across the screen, the toner particles are mixed with and adhere to the carrier particles. Subsequent cascading down the inclined plane :31 tends 'to improve mixing of the particles and contact between the carrier particles 11' and the toner particles 12 produces the tribo-electric forces necessary for the toner particles 12 to adhere to the carrier particles 11. If desired, obstacles, such as deflecting plates, can be located on the inclined plane 3lto further enhance mixing. Thus, the carrier particles 11 are returned to the depository 32 with substantially the same amount of toner particles as when they left. Of course, the roller 47 is rotated at such a speed as to supply toner particles 12 in an amount substantially equal to the amount of toner particles being used for printing upon the substrate 25.

Carrier particles 1 1, of course, continuously circulate during printing with any particular particleonly participating in perhaps every fourth or fifth print. While some are printing. some are being doctored, while other carrier particles are receiving additional toner particles. e

The printing machine 28 may be provided with any suitable source of power. In one operative embodiment of the invention, a suitable powersource was comprised of a So'rensen 230-3/12 P R & D high voltage DC. power supply, with an output variable from to 30 kilovolts. During a printing operation, a potential difference of approximately 12 kilovolts was maintained between the stencil screen 16 and the conductive belt electrode 30, with the screen being positive relative to the electrode 30. It is preferable that the stencil screen 16 be cleaned between each printing operation and, for such cleaning operation, a voltage of 12 kilovolts, or slightly less, was maintained with the screen being negative relative to-the base electrode 30.

As is illustrated in FIG. 2, high voltage sources 50, S1, 52 and 53 are provided. The high voltage source 52 maintains a potential difference of approximately 12 kilovolts between the base electrode .30 and the stencil screen 16, as set forth above. The voltage source 50 may be varied between 500 to l000 volts for maintaining the control grid 41 at such potential difference above the stencil screen 16 while the corona electrodes 40, which are comprised of two and three mil platinum wires, have been operated about kilovolts above the control grid 41 by high voltage source 51. The stencil screen 16 has been spaced approximately /2 inch above the conductive belt electrode 30, the control grid 41 has been spaced approximately /2 inch above the stencil screen 16, and the corona electrodes 40 have been spaced approximately /2 inch above'the control grid 41.

The control grid 41 is preferably comprised of a 16 X 16 mesh bronze screen woven of wire about 23 mils in diameter. The purpose of this control grid 41 is to control the flow of ions formed by the corona discharge, at thecorona electrodes 40, such that some of the ions formed in this discharge are carried to the substrate 25.

' If desired, suitable switches 55, 56 and 57 may be provided for simultaneously opening and closing the circuitry containing the high voltage sources 50, 51 and 52. Such switches 55, 56 and 57 may be connected together by a suitable mechanical connection which is illustrated by the dotted line 58.

In order to effect a cleaning operation of the stencil screen 16, between each printing operation, a high voltage source 53 may be supplied. The high voltage source 53 can establish a potential difference between the conductive base electrode 30 and the stencil screen 16 of 12 kilovolts, or slightly less, and is, therefore,

comparable to the high voltage source 52 except that a normally opened pushbutton 59 places the high voltage source 53 into the circuit such that the polarities of the conductive belt electrode30 and stencil screen '16 are reversed.

A further explanation of the cleaning operation is required. During the cleaning operation, the stencil screen 16 is maintained at the same polarity as the triboelectrically charged toner particles. In the cited example, this would be negative. The field intensity between these reen 16, and the belt 30, is sufficiently high that the developer particles oscillate vigorously between the screen and belt similarly to their motion during printing. However, the electric field is oppositely directed to that used for printing. The carrier particles mechanically dislodge accumulated toner from the screen when they strike and agitate it during the oscillations, Once dislodged, the electric field carries the toner away from the screen. Some toner can be removed with only a high intensity electric field of appropriate direction. However, this has not been found adequate for repetitive printing. Supplementing the field with the oscillating carrier particles, though, has been found to be completely adequate.

Some consideration must be given to both the intensity of the field during cleaning and the time permitted for the operation. If either of these are excessive, disadvantages may be encountered. The motion of the carrier particles during cleaning is quite comparable to that during printing. Thus, the carrier is delivering toner to the screen as it does during printing. Toner surrendered at the screen that has sufficient momentum and is appropriately directed will proceed on through the stencil apertures and produce printing. However, a field intensity sufficient to produce cleaning and yet insufficient to produce appreciable printing may be found. In general, it is desirable to clean with a lower field intensity than that used for printing so as to obtain this condition. Now, in addition, as long as the field is sufficiently intense to produce carrier oscillations, some printing necessarily occurs. Even with minimal carrier motion a high density print can be obtained if continued for a sufficient time, such as several minutes. However, the times actually used for cleaning typically are from 0.1 seconds to 0.4 seconds. By using a sufficiently small field intensity and a sufficiently short cleaning time, a screen can be exceptionally well cleaned and yet no readily discernable amount of toner be deposited on a substrate located in the printing position. Thus, objects may be moved into and out of the printing position while screen cleaning is occurring.

If the printed substrate is to be maintained in the printing location during the cleaning operation, then considerations of time and field intensity are less significant. However, by proper attention to the values permitted these variables, one can obtain the benefits of a cleaning operation that is substantially free of simultaneous printing.

Illustrated in FIG. 3 is a feature of the present invention wherein the carrier particles 11 can be used to clean the printing machine 28 at the end of a printing task. A continuous electrode 60, havinga continuous conductive surface 61, may be substituted for the stencil screen 16 and can be used to remove toner particles from the carrier particles 11. This is done by maintaining the cleaning electrode 60, by the high voltage source 62, at substantially the same potential as the stencil screen 16 when printing is occurring. The electric field between the cleaning electrode 60 and the conductive belt electrode 30 causes the carrier particles to oscillate back-and-forth between them. As toner particles are freed by carrier collisions, they are attracted to the cleaning electrode 60. In this manner, toner particles may be removed from the carrier particles in substantial quantities. The collected toner particles can be continuously removed from the cleaning electrode 60, by a suitable scraper 63, and deposited in a toner recovery bin 64.

Cleaned carrier particles 11 are recirculated through the printing machine 28 and will pick up any toner particles that may have collected on surfaces in the printing machine itself. By maintaining this cleaning and recirculating operation for approximately several minutes, substantially all cleaning required to change toner colors can be automatically and expeditiously accomplished. Of course, during brush 47 is maintained at rest so as not to add additional toner particles to the system.

While preferred forms and arrangement of parts have been shown inillustrating the invention, it is to be clearly understood that various changes in details and arrangement of parts may be madewithout departing from the spirit and scope, of the invention, as defined in the appended claimed subject matter.

We claim:

1. In an electrostatic apparatus utilizing carrier particles and toner particles, a base electrode, means for depositing carrier particles and toner particles on to said base electrode, a cleaning electrode, means for transporting said carrier particles and saidtoner particles from said base electrode to said cleaning electrode, and means for causing said toner particles to adhere to said cleaning'electrode and said carrier particles to be expelled from said cleaning electrode.

25in an electrostatic apparatus as'defined in claim 1 1 wherein said base electrode. comprises a conductive belt, and said carrier particles are formed of conductive material.

' 3.111 an electrostatic apparatus as defined in claim 1 wherein said cleaning electrode has a continuous surface of conductive material.

4. In an electrostatic apparatus as defined in claim 1 wherein said base electrode and said cleaning electrode each have a continuous surface and each are comprised of conductive material, said transporting means comprising asource of electrical energy, said base electrode being; connected to oneside of said source and said cleaning electrode being connected to the other side of said source for creating an electric field between said base electrode and said cleaning electrode.

5. In an electrostatic apparatus as defined in claim 1 wherein said cleaning electrode comprises a rotatable drum having at least the surface thereof being formed of conductive material, and means for removing toner particles from said surface.

6. In an electrostatic apparatus as defined in claim 1 wherein said means for depositing said carrier particles and toner particles on said base'electrode comprises a magneticdrum and a magnetic roller, said magnetic roller having a more intense magnetic field than said magnetic drum, said base electrode comprising an end less belt, said belt having a portion thereof disposed between'said magnetic drum and said magnetic roller, whereby said carrierparticles and said toner particles are attracted'from said magnetic drum toward said magnetic roller and deposited upon said endless belt.

7. In an apparatus as defined in claim 5 wherein an inclined plane extends downwardly from said endless belt toward said magnetic drum, whereby particles cascading down said inclined plane are directed toward said magnetic drum and attracted by its magnetic field.

8. In an electrostatic printing apparatus, a base electrode and a stencil screen, a developer comprising toner particles and conductive carrier particles, means for depositing said developer upon said base electrode, means for establishing an electric fieldbetween said base electrode and said stencil screen for causing said carrier particles to oscillate within said electric field bethe cleaning operation, the

tween said base electrode and said stencil screen, and apparatus means for causing toner particles to be returned from said stencil screen to said base electrode subsequent to a printing operation, said apparatus 5 means comprising means for reversing the direction of said electric field established between said base electrode and said stencil screen for oscillating said carrier particles between said base electrode and said stencil screen for dislodging toner particles from said screen whereby the reversed direction of said electric field causes the dislodged toner particles to be attracted toward said base electrode.

9. In an electrostatic printing apparatus as defined in claim 8, said apparatus means being further defined as including means for reducing the field intensity of said electric field.

10. In an electrostatic printing apparatus, a base electrode and a stencil screen, a developer comprising electrically non-conductive toner particles and electrically 20 conductive carrier particles, means for depositing said developer upon'siad, base electrode, means for 'establishing an electric field between said base electrode and said stencil screen for causing said carrier particles to oscillate within said electric field between said base electrode and said stencil screen, and apparatus means for causing toner particles to be returned from said stencil screen to said base electrode subsequent to a printing operation.

11. In an electrostatic printing apparatus as defined in claim wherein said stencil screen has openings therein for defining a pattern; and'said openings, said toner particles and said carrier particles being of respective sizes for allowing said toner particles to pass through said openings and precluding passage of said carrier particles through saidopenings.

12. In an electrostatic printing apparatus as defined in claim 10 wherein said apparatus means comprises means for reversing the direction of said electric field and means for agitating said stencil screen for dislodging toner particles therefrom whereby the dislodged toner particles are carried by said reversed electric field to said base electrode.

13. In an electrostatic printing apparatus as defined in claim 12 wherein said stencil screen is agitated by bombardment of carrier particles against said stencil screen.

14. In an electrostatic screen printing apparatus, first means for cleaning electrically non-conductive toner 50 particles from a stencil screen subsequent to a printing operation, said first means including said stencil screen and a base electrode, a voltage source for establishing an electric field between said stencil screen and said base electrode, and second means for causing toner particles to be freed from said stencil screen and attracted toward said base electrode. 15. In an electrostatic screen printing apparatus as defined in claim 14 wherein said second means includes a third means for agitating said stencil screen for dislodging toner particles therefrom.

16. Apparatus as in claim 15 wherein said third means comprises a plurality of electrically conductive particles for bombarding said stencil screen. 

1. In an electrostatic apparatus utilizing carrier particles and toner particles, a base electrode, means for depositing carrier particles and toner particles on to said base electrode, a cleaning electrode, means for transporting said carrier particles and said toner particles from said base electrode to said cleaning electrode, and means for causing said toner particles to adhere to said cleaning electrode and said carrier particles to be expelled from said cleaning electrode.
 2. In an electrostatic apparatus as defined in claim 1 wherein said base electrode comprises a conductive belt, and said carrier particles are formed of conductive material.
 3. In an electrostatic apparatus as defined in claim 1 wherein said cleaning electrode has a continuous surface of conductive material.
 4. In an electrostatic apparatus as defined in claim 1 wherein said base electrode and said cleaning electrode each have a continuous surface and each are comprised of conductive material, said transporting means comprising a source of electrical energy, said base electrode being connected to one side of said source and said cleaning electrode being connected to the other side of said source for creating an electric field between said base electrode and said cleaning electrode.
 5. In an electrostatic apparatus as defined in claim 1 wherein said cleaning electrode comprises a rotatable drum having at least the surface thereof being formed of conductive material, and means for removing toner particles from said surface.
 6. In an electrostatic apparatus as defined in claim 1 wherein said means for depositing said carrier particles and toner particles on said base electrode comprises a magnetic drum and a magnetic roller, said magnetic roller having a more intense magnetic field than said magnetic drum, said base electrode comprising an endless belt, said belt having a portion thereof disposed between said magnetic drum and said magnetic roller, whereby said carrier particles and said toner particles are attracted from said magnetic drum toward said magnetic roller and deposited upon said endless belt.
 7. In an apparatus as defined in claim 5 wherein an inclined plane extends downwardly from said endless belt toward said magnetic drum, whereby particles cascading down said inclined plane are directed toward said magnetic drum and attracted by its magnetic field.
 8. In an electrostatic printing apparatus, a base electrode and a stencil screen, a developer comprising toner particles and conductive carrier particles, means for depositing said developer upon said base electrode, means for establishing an electric field between said base electrode and said stencIl screen for causing said carrier particles to oscillate within said electric field between said base electrode and said stencil screen, and apparatus means for causing toner particles to be returned from said stencil screen to said base electrode subsequent to a printing operation, said apparatus means comprising means for reversing the direction of said electric field established between said base electrode and said stencil screen for oscillating said carrier particles between said base electrode and said stencil screen for dislodging toner particles from said screen whereby the reversed direction of said electric field causes the dislodged toner particles to be attracted toward said base electrode.
 9. In an electrostatic printing apparatus as defined in claim 8, said apparatus means being further defined as including means for reducing the field intensity of said electric field.
 10. In an electrostatic printing apparatus, a base electrode and a stencil screen, a developer comprising electrically non-conductive toner particles and electrically conductive carrier particles, means for depositing said developer upon siad base electrode, means for establishing an electric field between said base electrode and said stencil screen for causing said carrier particles to oscillate within said electric field between said base electrode and said stencil screen, and apparatus means for causing toner particles to be returned from said stencil screen to said base electrode subsequent to a printing operation.
 11. In an electrostatic printing apparatus as defined in claim 10 wherein said stencil screen has openings therein for defining a pattern; and said openings, said toner particles and said carrier particles being of respective sizes for allowing said toner particles to pass through said openings and precluding passage of said carrier particles through said openings.
 12. In an electrostatic printing apparatus as defined in claim 10 wherein said apparatus means comprises means for reversing the direction of said electric field and means for agitating said stencil screen for dislodging toner particles therefrom whereby the dislodged toner particles are carried by said reversed electric field to said base electrode.
 13. In an electrostatic printing apparatus as defined in claim 12 wherein said stencil screen is agitated by bombardment of carrier particles against said stencil screen.
 14. In an electrostatic screen printing apparatus, first means for cleaning electrically non-conductive toner particles from a stencil screen subsequent to a printing operation, said first means including said stencil screen and a base electrode, a voltage source for establishing an electric field between said stencil screen and said base electrode, and second means for causing toner particles to be freed from said stencil screen and attracted toward said base electrode.
 15. In an electrostatic screen printing apparatus as defined in claim 14 wherein said second means includes a third means for agitating said stencil screen for dislodging toner particles therefrom.
 16. Apparatus as in claim 15 wherein said third means comprises a plurality of electrically conductive particles for bombarding said stencil screen. 